1. Field of the Invention
The present invention relates to the fabrication of structural units from titanium aluminide materials, and more particularly to a process and an apparatus for selectively joining titanium aluminide sheets to form a metallic structure which can then be superplastically expanded to form a sandwich structure of a desired shape.
2. Discussion of the Known Prior Art
Titanium aluminide materials are well known in the metallurgical arts as materials which exhibit excellent high-temperature strength and oxidation and creep resistance.
Titanium aluminides, like other titanium alloys, are metals which are relatively brittle and difficult to process and/or fabricate at or near room temperatures. One fabrication technique which has found widespread utility in the fashioning of structures for various industries is diffusion bonding.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to cause co-mingling of atoms at the joint interface.
The application of pressure accomplishes a plastic flow of the material to fill normal void areas. If the pressure is too low, small voids will remain at the joint interface and the joint strength will be less than the maximum attainable. The application of pressure also breaks up the surface oxides and surface asperites so as to present clean surfaces for bonding.
The elevated temperatures used for diffusion bonding serve to accelerate diffusion of atoms at the joint interfaces as well as to provide a metal softening which aids in surface deformation thereby allowing more intimate contact for atom bonding and movement across the joint interface.
With practice of this technique, it has become accepted that diffusion bonding of titanium aluminide materials must be conducted in a controlled environment in order to ensure cleanliness of the materials which are particularly sensitive to oxygen, nitrogen, and water vapor content in the air at elevated temperatures. Unless titanium aluminide is protected during this bonding process, it becomes embrittled and its integrity is destroyed.